Study of accelerated ion energy and spatial distribution with variable thickness liquid crystal targets

We report on laser-based ion acceleration using freely suspended liquid crystal film targets, formed with thicknesses varying from 100 $nm$ to 2 $\mu m$ for this experiment. Optimization of Target Normal Sheath Acceleration (TNSA) of protons is shown using a 1 $\times$ $10^{20}$ $W/cm^2$, 30 fs laser with intensity contrast better than $10^{-7}:1$. The optimum thickness was near 700 $nm$, resulting in a proton energy maximum of 24 $MeV$. Radiochromic film (RCF) was employed on both the laser and target normal axes, revealing minimal laser axis signal but a striking ring distribution in the low energy target normal ion signature that varies with liquid crystal thickness. Discussion of this phenomenon and a comparison to similar observations on other laser systems is included.